The formula with 18 substituted
L/100km = 235.214583 ÷ 18 = 13.07
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The MPG to L/100km Converter explains where each comes from and gives a longer table.
The same 18 mpg (US) is 21.62 mpg on the imperial gallon, 13.07 L/100km and 7.65 km/L. If your 18 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 15.69 L/100km instead.
What 18 mpg (US) means in practice
18 mpg (US), 13.07 L/100km, is the combined rating of many full-size pickups with turbocharged V6 engines. It is respectable for the class and poor for anything smaller.